Many different forms of connectors are known for use with wires. Some of these connectors are attached to the end of a wire, and crimped on. The crimp, that is usually a pressure or other kind of connection between the elements of the wire and the elements of the connector itself, hold the wire and the connector together.
Strain relief may be used in certain types of wires, so that pulling on the wire does not correspondingly cause the wire to be removed from the connector. However, in many connectors, there is simply no adequate method of strain relief.
For example, many network connectors, such as those used with RJ 45 type connections, allow all strain relief to simply be done by the connection between the connector and the wire. This makes it possible for the wire to fail in various ways, e.g., especially when the wire is used in a high stress situation.